Vacuum pump components without conversion layers

ABSTRACT

The invention relates to vacuum pump components without conversion layers that are made of valve metals and alloys thereof.

The invention relates to vacuum pump components without conversionlayers that are made of valve metals and alloys thereof.

DE 101 63 864 A1 relates to a process for the coating of objects made ofvalve metals or their alloys with a thin barrier layer consisting of themetal and an oxide ceramic layer provided thereon whose surface has beencoated with fluoropolymers, characterized in that the fluoropolymers areintroduced into the capillary system of the oxide ceramic layer in theform of a solution by vacuum impregnation, followed by removing thenon-wetting portions of the solution and drying. Accordingly, the groupof valve metals includes aluminum, magnesium, titanium, niobium orzirconium and their alloys. In addition, this specification also definesfurther components of vacuum pumps made of valve metals, such as rotorsand stators of turbo-molecular pumps.

In this document and also within the scope of the present invention,“aluminum and its alloys” means ultrapure aluminum and the alloys AlMn;AlMnCu; AlMg₁, AlMg_(1.5); E-AlMgSi; AlMgSi_(0.5); AlZnMgCu_(0.5);AlZnMgCu_(1.5); G-AlSi_(1.2); G-AlSi₅MG; G-AlSi₈Cu₃; G-AlCu₄Ti;G-AlCu₄TiMg. Further, in addition to pure magnesium, in particular, themagnesium cast alloys with the ASTM designations AS41, AM60, AZ61, AZ63,AZ81; AZ91, HK31, QE22, ZE41, ZH62, ZK51, ZK61, EZ33, HZ32 and thewrought alloys AZ31, AZ61, AZ80, M1 ZK60, ZK40 are suitable for thepurposes of the invention. Further, pure titanium and also titaniumalloys such as TiAl₆V₄, TiAl₅Fe_(2.5) and others may be employed.

In DE 101 63 864 A1, the oxide ceramic layer is essentially formed by aconversion layer of the surface of the component, so that in practicepart of the substrate material is lost and converted to the oxidationbarrier layer.

In addition, it is known that conventional anodization layers,plasma-chemical anodization methods (KEPLA-Coat®, KERONITE® and othermethods) are known. Also, it is known to nickel-coat the above mentionedvalve metals.

All the above mentioned coating methods enable true-contour layers to beformed. However, all these layer systems have specific disadvantages invacuum- technological application. Thus, the anodization methods includemore or less pronounced pore structures that limit the corrosionprotection. The electroless nickel layers have so-called “pinholes”,which at least require a greater layer thickness in order to minimizethe number and size of the pinholes. Also, the tribological behavior ofelectroless nickel layers, especially under vacuum, is insufficientbecause such layers tend to cause cold welding during crashs.

Alternative coatings for components of vacuum pumps are also described,for example, in DE 10 2005 040 648 A1. Objects made of valve metals,especially components of vacuum pumps with an oxide ceramic layer formedfrom the metal that has a thin barrier layer as interface towards themetal are provided with another polymeric layer formed on the basis ofparylenes. In this respect, DE 10 2005 040 648 A1 is included herein byreference in its entirety with respect to the composition of thissurface coat and the application method.

WO 03/029529 A1, WO 2006/047501 A2 and WO 2006/047526 A2 describe thepreparation of an object with a ceramic coating of titania and/orzirconia that is resistant to corrosion, heat and abrasion, applied tosaid object made of aluminum and/or titanium by direct-current oralternating-current anodization. The objects underlying this prior artmethod are not specified. Also, there is no information about chemicalresistance, especially resistance to citric acid or hydrochloric and/orhydrofluoric acid vapors.

Accordingly, the object of the present invention is to provide vacuumpump components without conversion layers, made of valve metals or theiralloys, that, in addition to having corrosion, heat and abrasionresistance, include a coating without a conversion layer that isproduced by electroplating and is additionally resistant to chemicals,especially resistant to citric acid or hydrochloric acid vapors. This isparticularly important in the production of components of vacuum pumpsthat, especially in vacuum technologies, come into contact withaggressive gases, such as HCl and/or HF vapors/gases.

In a first embodiment, the above object is achieved by vacuum pumpcomponents without conversion layers, made of valve metals or theiralloys, characterized in that their surfaces have a coating of at leastone oxide and/or oxyfluoride of an element of the group consisting ofboron, germanium, aluminum, magnesium, titanium, niobium, hafnium and/orzirconium and mixtures thereof, produced by electroplating and having alayer thickness within a range of from 5 to 50 μm.

The solution to the above problem makes use of the true-contour coveringof surfaces that is possible in known conversion layers, such asKEPLA-Coat®, or anodization layers on the one hand. However, accordingto the present invention, it is essential that no substrate material islost by conversion, i.e., no conversion layer is produced. Thus, ifneeded, the coating can be repeated any number of times withoutsubstrate losses, which is highly important for maintenance, inparticular.

Details of process technology can be seen in detail from the abovementioned publications WO 03/029529 A1, WO 2006/047501 A2 and WO2006/047526 A2. In this respect, these documents are also includedherein by reference in their entirety.

Because of the high deposition rates, the exposure times are reduced toabout one third as compared to the usual anodization methods, and evento one sixth (quarter) as compared to the above mentioned KEPLA Coat®method. Thus, a significant economical advantage is provided. Inaddition, it could be observed that no edge effect has occurred with thevacuum pump components without conversion layers, made of valve metalsand their alloys, prepared according to the invention. This property, inparticular, has not been known from the above mentioned documents andthus represents a surprising advantage of the present invention.

As compared to the layers prepared by anodization methods or by theKEPLA Coat® method, a higher resistance to abrasive wear is obtained.The deposited layers may have a hardness of about 700 HV.

According to the invention, significant advantages in corrosionprotection over the known layer systems could be achieved. This includesprotection against citric acid and hydrochloric acid, in particular. Itis known that anodization layers are sensitive to the action of citricacid, while the KEPLA Coat® layers have no sufficient stability tohydrochloric acid.

It has been found that the basic service life of the electrolyte can beset by analytical monitoring and optionally replenishing oversignificantly longer periods than those required with the previouslyknown methods for the coating of components of vacuum pumps made ofvalve metals and their alloys. In contrast, the electrolyte of KEPLACoat® must be discarded depending on usage because of contaminationsoriginating from the starting material. This similarly applies toelectrolytes of anodization layers.

Components of vacuum pumps made of valve metals and their alloysaccording to the invention include, in particular, rotors, stators,stator disk halves, helical stages, housings and bearing shells.

In accordance with the prior art, the term “valve metals” hereinincludes metals of the group of aluminum, magnesium, titanium, niobiumand/or zirconium and their alloys. The specific alloys of aluminum,magnesium and titanium as mentioned in the introductory part of thedescription are also particularly preferred according to the presentinvention.

It is particularly preferred to select at least one oxide and/oroxyfluorides of the group consisting of aluminum, titanium and/orzirconium for the surface coating. These are best suitable for realizingthe advantages of the present invention.

In one embodiment of the present invention, the thickness of the surfacecoating is from 5 to 50 μm. It is particularly preferred according tothe present invention that the thickness of the surface coating is from15 to 30 μm. If the thickness of the surface coating is selected toothin, sufficient protection against corrosion, heat, abrasion andchemicals cannot be ensured. In contrast, if the thickness of thesurface coating is selected too large, the corresponding coatings willtend to chip off. In addition, correspondingly thick coatings areeconomically inefficient.

Another embodiment of the present invention relates to a process forpreparing vacuum pump components without conversion layers, made ofvalve metals or their alloys, that are produced by electroplating,characterized by

-   -   (a) providing an anodizing solution containing, in addition to        water, at least one other component selected from the group of        water-dispersible complex fluorides and oxyfluorides of elements        of the group consisting of boron, germanium, aluminum,        magnesium, titanium, niobium, hafnium and/or zirconium and        mixtures thereof;    -   (b) contacting a cathode with said anodizing solution;    -   (c) inserting the components as anodes into said anodizing        solution; and    -   (d) applying a voltage between the anode and cathode to apply a        surface coating to said components.

In principle, this process is already known from the mentioned documentsWO 03/029529 A1, WO 2006/047501 A2 and WO 2006/047526 A2. The presentinvention is distinguished therefrom by the selected components ofvacuum pumps made of valve metals and their alloys.

EXAMPLES Example 1

A sample sheet of AlMgSi₁ with dimensions of 100×50×1.5 mm was subjectedto anodic coating at 400 volts for 5 minutes in an electrolyte asdescribed in WO 03/029529 A1, WO 2006/047501 A2 and WO 2006/047526 A2within 5 minutes. The determined layer thickness was about 10 μm.

Example 2

A sample sheet as described in Example 1 was coated in an analogous waywithin 10 minutes. The determined layer thickness was about 12 μm.

Example 3

The sample sheets coated according to Examples 1 and 2 were exposed to ahydrochloric acid atmosphere formed above a bath containing 15% byweight hydrochloric acid. The oxide ceramic layer on the sample sheetswas examined for chipping off after test durations of 144 hours and 300hours. The oxide ceramic layer on the sample sheets was still intactafter this exposure time.

Example 4

The sample sheets coated according to Examples 1 and 2 were exposed tocitric acid solutions having concentrations of 2%, 3.5% and 5%. Theoxide ceramic layer on the sample sheets was examined for chipping offafter a test duration of 90 hours. The oxide ceramic layer on the samplesheets was still intact after this exposure time.

1. Vacuum pump components without conversion layers, made of valvemetals or their alloys, characterized in that their surfaces have acoating of at least one oxide and/or oxyfluoride of an element of thegroup consisting of boron, germanium, aluminum, magnesium, titanium,niobium, hafnium and/or zirconium and mixtures thereof, produced byelectroplating and having a layer thickness within a range of from 5 to50 μm.
 2. The components according to claim 1, including rotors,stators, stator disk halves, helical stages, housings and bearingshells.
 3. . The components according to claim 1, characterized in thatsaid valve metal is selected from aluminum, magnesium, titanium, niobiumand/or zirconium and their alloys.
 4. The components according to claim1, characterized in that said coating consists of at least one oxideand/or oxyfluoride of the group consisting of aluminum, titanium and/orzirconium.
 5. The components according to claim 1, characterized in thatthe thickness of the surface coating is from 15 to 30 μm.
 6. A processfor preparing vacuum pump components without conversion layers, made ofvalve metals or their alloys, that are produced by electroplating,characterized by (a) providing an anodizing solution containing, inaddition to water, at least one other component selected from the groupof water-dispersible complex fluorides and oxyfluorides of elements ofthe group consisting of boron, germanium, aluminum, magnesium, titanium,niobium, hafnium and/or zirconium and mixtures thereof; (b) contacting acathode with said anodizing solution; (c) inserting the components asanodes into said anodizing solution; (d) applying a voltage between theanode and cathode to apply a surface coating to said components.